One research paper published in New Phytologist describes the independent relationship between leaf mechanical strength and photosynthetic capacity across 57 woody species with contrasting light requirements in Dinghushan Biosphere Reserve. 

Forest species can be clarified as either 'light demanding' or 'shade tolerant' according to their probability of seedling establishment and survival in the understory. Shade-tolerant species are commonly defined as those with equal survival in gaps and shade, whereas light-demanding species are those incapable of regeneration in deep shade. 

Leaves are subject to a range of physical hazards, both biotic (e.g. herbivory) and abiotic (e.g. wind, mechanical damage from neighbouring branches). Tougher leaves tend to have longer leaf lifespan. 

Despite the considerable attention paid to differences between light-demanding and shade-tolerant species, there have been few, if any, studies that have simultaneously reported data for leaf biomechanics and photosynthetic traits. 

A research group led by Prof. YE Qing from South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS) examined leaf traits and trait relationships in a broad sample of light-demanding and shade-tolerant species from a subtropical evergreen forest in Southern China. 

They found that shade-tolerant species had longer leaf lifespan, higher leaf mechanical strength but lower photosynthetic rates, and exhibited longer leaf lifespan for a given leaf mass per area or mechanical strength than light-demanding species.

Depending on the traits and the basis of expression (per area or per mass), leaf mechanical resistance and photosynthetic capacity were either deemed unrelated, or only weakly negatively correlated.  

"We found only weak support for the proposed trade-off between leaf biomechanics and photosynthesis among co-occurring woody species. This suggests there is considerable flexibility in these properties, and the observed relationships may result more so from trait coordination than any physically or physiologically enforced trade-off." said Dr. HE Pengcheng, the first author of this study.